This invention relates to the field of process control. More particularly the invention relates to analyzing defect data from historical and tolerance perspectives.
Many processes, such as wafer processing in the semiconductor industry, utilize testing of the process material at various points in the process. There are many reasons why the process material is tested. One reason is to determine whether the process material meets the design specifications required for the process material to be of value. Another reason is to determine whether the process itself, such as the equipment and methods used in the process, is inherently capable of producing process material that meets the design specifications.
In the specific example of wafer processing, as introduced above, such testing is typically performed according to a method as described below. An operator or technician takes readings on the wafer either by direct inspection, such as under a microscope, or by processing the wafer through a specialized test instrument. Regardless of the specific method used, the operator gathers property information in regard to one or more process characteristics that are determinable by inspection of the wafer by one or more of the methods mentioned above.
The operator prepares a summary of the test that was performed and the results that were obtained in regard to the process characteristics sensed during the test. Depending on factors such as the complexity of the test, the ambiguity of the results, or the number of process characteristics sensed, the operator may spend forty-five minutes or more preparing such a summary. The operator then checks the summary results against published specification limits for the process characteristics sensed, such as statistical process control charts. The statistical process control charts may be in either hard copy form, such as might be posted on the processing or testing equipment itself, or in soft copy form, such as resident in the processing or testing equipment controllers, or in some other computerized device.
The operator makes a determination as to whether the process material, in this example the wafer, passes or fails the test based on the comparison of the test results for the process characteristics and the published specification limits. This is often not as easy to do as it may initially appear. For example, the property information for the process characteristics may indicate failure of the wafer based on the specification limits as to some aspects of the process characteristics, but may also indicate acceptance of the wafer based on the specification limits as to other aspects of the process characteristics. At times, different tests of the same process characteristics may give differing results, some which tend to indicate failure of the wafer, and others which tend to indicate acceptance of the wafer. In addition, test results may be at the borderline of the published specification limits, and thereby fail to indicate either failure of the wafer or acceptance of the wafer.
In such borderline cases, or in cases where the wafer fails the test, the operator may further review the results of the test, or perform additional tests on the wafer to generate additional data to help form a more clear indication of whether the wafer should be failed or passed. What additional tests to perform is typically left to the discretion of the operator, and varies widely from person to person. The additional test information, together with the original test results, are then held for the review of an engineer who has responsibility for the specific process steps and tests involved. The wafer may be on hold for a day or more before the designated engineer has the opportunity to review the test results.
Upon review of the test results, the engineer makes a decision as to whether the wafer is passed or failed. Because the engineer tends to make a subjective decision based upon the personal experience and judgment of the engineer as formed over a period of time, the decisions made by different engineers tend to be different one from the other. For example, one group of engineers may believe that a first given set of factors takes precedent over a second given set of factors when forming a decision, and another group of engineers may believe that the second given set of factors takes precedent over the first given set of factors when forming a decision. Further, the same engineer may make seemingly inconsistent decisions when faced with the same criteria, depending upon his feelings or other apparently irrelevant criteria existing at the time of the decision.
Further, the engineer may take other steps in addition to passing or failing the wafer based on the results of the test. For example, the engineer may modify the published specification limits based on the results of the test. The modification made by the engineer may be different than and even contrary to the modification that may be made by a different engineer, as described above. The change to the specification made by the engineer may not be brought to the immediate attention of all of the operators performing the test. In this manner, operators may mistakenly pass or fail wafers inconsistently for a period of time, until all of the operators are made aware of the new specification limits. Further, the engineer may change the process settings for the wafers that are yet to be processed, based on the results of the test.
Thus, there are many problems inherent with the system as described above. For example, because of the very manual nature of the system, only a relatively limited amount of test limits can be analyzed by the operators performing the tests. Additionally, because the operator has the responsibility to compare the results of the test to the specification limits, there is a likelihood that the operator will make comparison errors and select an incorrect disposition for the wafer. Further, all decisions made in regard to the wafer are based on the differing individual experiences and attitudes of the people who make the decisions. Thus, not only is such a system extremely time consuming, it also produces inconsistent results.
What is needed, therefore, is a system for automating, broadening and standardizing the criteria base by which wafer pass and fail decisions, specification limit change decisions, and process control change decisions are made.
The above and other needs are met by a computerized system for analyzing property information associated with a process characteristic of a process unit. A database contains historical information relating to previously compiled property information associated with the process characteristic. Secure input means receive limit criteria, compilation criteria, selection criteria, manipulation criteria, and disposition criteria from a restricted source. Computer means mathematically determine a limit based upon the limit criteria. Open input means receive the property information associated with the process characteristic of the process unit from multiple test locations.
Compilation means selectively add to the database of historical information the property information associated with the process characteristic of the process unit received from the multiple test locations based upon the compilation criteria. The computing means also select at least a portion of the property information associated with the process characteristic of the process unit based upon the selection criteria. In addition, the computing means manipulate the selected property information associated with the process characteristic of the process unit based upon the manipulation criteria.
Comparison means compare the manipulated property information associated with the process characteristic of the process unit against the limit. Output means indicate a first disposition of the process unit when the manipulated property information associated with the process characteristic of the process unit violates the limit, based upon the disposition criteria. The output means indicate a second disposition of the process unit when the manipulated property information associated with process characteristic does not violate the limit, based upon the disposition criteria.
Thus, the computerized system uses the knowledge that has been acquired from previously processed process units, as contained in the database of historical information, to indicate first and second dispositions, depending upon how the property information associated with the process characteristic of the process unit compares to the limit. In this manner the restricted source, as the provider of the various criteria, is the sole seat of the decision making authority, and more consistent, better reasoned decisions are made. Thus, others with less training, poorer judgment, or emotionally clouded reason are not in a position to make spurious decisions in regard to either the disposition of the lot or control of the process. The first and second dispositions, based upon the disposition criteria, can contain explicit instructions in regard to these or many other factors. Further, by concentrating the decision making authority to a restricted source, as administered through the computerized system, the time previously lost by multiple people attempting to make the difficult decisions inherent in process testing and analysis is reclaimed.
In various preferred embodiments of the system the limit is a scrap limit where the first disposition is to scrap the process unit and the second disposition is to pass the process unit. In an alternate embodiment the limit further comprises a learning limit where the first disposition is to gather additional property information associated with the process characteristic of the process unit and the second disposition is to pass the process unit.
In a most preferred embodiment, which is a blend of the two alternate embodiments described above, the limit is both a scrap limit and a learning limit. In this embodiment with two limits, the first disposition in regard to the scrap limit is to scrap the process unit, and the second disposition in regard to the scrap limit is to compare the manipulated property information associated with the process characteristic of the process unit against the learning limit. The first disposition in regard to the learning limit is to gather additional property information associated with the process characteristic of the process unit, and the second disposition in regard to the learning limit is to pass the process unit.
Preferably, the output means indicate specific tests to be accomplished on the process unit, so as to gather additional property information associated with the process characteristic of the process unit when the manipulated property information associated with the process characteristic violates the limit and the first disposition of the process unit is indicated.
Also in a preferred embodiment, the computing means mathematically determine a plurality of limits based upon the limit criteria, make a plurality of selections from the property information associated with the process characteristic of the process unit based upon the selection criteria, and independently manipulate the plurality of selections based upon the manipulation criteria. In this embodiment, the comparison means independently compare the plurality of manipulated selections against the plurality of limits, and the output means indicate a first disposition of the process unit and second disposition of the process unit for each independent comparison of the plurality of manipulated selections against the plurality of limits.
In a method for selecting a disposition of a process unit according to the present invention, a database is compiled of historical information relating to previously sensed property information associated with a process characteristic of the process unit. A limit is determined, based upon limit criteria. Property information associated with the process characteristic of the process unit is sensed. The sensed property information associated with the process characteristic of the process unit is selectively added to the database of historical information, based upon compilation criteria. At least a portion of the sensed property information associated with the process characteristic of the process unit is selected, based upon the selection criteria.
The selected property information associated with the process characteristic of the process unit is manipulated based upon manipulation criteria. Then, the manipulated property information associated with the process characteristic of the process unit is compared against the limit. A first disposition of the process unit is selected when the manipulated property information associated with the process characteristic of the process unit violates the limit. A second disposition of the process unit is selected when the manipulated property information associated with the process characteristic of the process unit does not violate the limit.